1. Technical Field
The present invention relates to a semiconductor device and a method for manufacturing the same, in particular to a semiconductor which is suitably applied to a filed effect transistor formed on a Silicon On Insulator (SOT) substrate.
2. Related Art
The utility of field effect transistors formed on a SOI substrate is attracting attention due to easiness of element isolation, characteristic of latch-up free, small source/drain junction capacitances etc. In particular, since fully depleted type SOI transistors can be operated with low power consumption and at high speeds, researches on operating SOI transistors in a completely depleted mode are actively conducted. It should be noted that, as a SOI substrate, for example, a Separation by Implanted Oxygen (SIMOX) substrate, a laminated substrate or the like may be used, as described in JP-A-2002-299591 and JP-A-2000-124092.
Further, JP-A-2002-299591 discloses a method for forming a SOI transistor at a low cost by forming a SOI layer on a bulk substrate. According to the method disclosed in JP-A-2002-299591, a Si/SiGe layer is formed on a Si substrate, and only a SiGe layer is selectively removed by utilizing the difference in selectivity between Si and SiGe, thereby forming a void portion between the Si substrate and the Si layer. Then, by performing thermal oxidation of Si exposed into the void portion, a SiO2 layer is embedded between the Si substrate and the Si layer and a BOX layer is formed between the Si substrate and the Si layer.
However, in order to manufacture a SIMOX substrate, highly concentrated oxygen needs to be ion-implanted in a silicon wafer. Further, in order to manufacture a bonded wafer, two silicon wafers need to be bonded together, and then the surface of the silicon wafer needs to be polished. For this reason, there is a problem in that the cost of SOI transistors increases compared to field effect transistors that are formed in bulk semiconductor.
Furthermore, there is a problem in that the ion implantation and polishing cause a large variation in film thickness of a SOI layer, so it becomes difficult to stabilize the characteristics of field effect transistors when the SOI layer is made into a thin film for manufacturing fully depleted type SOT transistors.
Moreover, as disclosed in T. Sakai et al. “Separation by Bonding Si Islands (SBSI) for LSI Application”, Second International SiGe Technology and Device Meeting, Meeting Abstract, pp. 230-231, May (2004), the film thickness of the SOT layer on the BOX layer is uniform, so there arises a problem in that the drain breakdown voltage may be deteriorated or electrostatic breakdown level may become lower when the film thickness of the SOI layer in a channel region is made to be thinner in order to obtain a steep subthreshold slope. On the other hand, when the film thickness of the SOI layer in the channel region is thicker in order to ensure the drain breakdown voltage, a subthreshold slope of the drain current are deteriorated, which becomes a hindrance to a low voltage operation of transistors and leads to increase in leakage current in OFF condition, thereby increasing power consumption of the device while operating or waiting.